Say I had a PHP page with a calculator interface, but I want to transfer the sum to Windows Calculator.exe and then get a reply from the application back to my PHP page, how do you go about doing this?
I understand that the sum could easily be done in PHP and it would be pointless using another application, but I just wondered how it would be done and will be helpful to know for my next project. I couldn't find any relevent exisiting questions but the search criteria for this isn't easy!
Thanks
Applications can communicate using an Application Programming Interface (API). When writing a program, the developer can choose to expose certain functions to allow other programs to interact with them. By only exposing certain functions, and in a certain way, the developer allows people to use his application in a safe way without accidentally (or intentionally) doing destructive things.
An API may be exposed in several ways, depending on the languages involved. In a .NET environment, for instance another .NET program could add a reference to an API dll created for this purpose. Another example is a web service API which can be communicated with using HTTP requests. If the API uses a common cross language technique such as JSON or XML to input and output data, a program in any language may be able to communicate with it.
In addition to what @razethestray said, applications can also talk to each other by means of so-called RPC interfaces, which stands for Remote Procedure Calls.
However, RPC itself is a broad term. Actually there are plenty of ways how an RPC could be implemented. For example, Microsoft's DCOM relies heavily on RPC. On the other hand, a SOAP (web) server call is considered an RPC as well. the one thing that they all have in common is, that a client calls a piece of code in a remote (server) process through a well-defined API to achive something.
As you can imagine, it is a good idea and highly recommended to use existing frameworks for such a task, simply to hide and manage the complexity that comes with the infrastructure needed for this. Those frameworks and in some cases built-in IDE support (e.g. for SOAP) make an RPC essentially look like a normal function/method call. In fact, behind the scenes there may be happen a lot, especially when the other process is on a remote machine.
PS: That's a quite sophisticated topic for starters.
